In recent years, progress has been made in architecting optical network communications using WDM (Wavelength Division Multiplexing) techniques for multiplexing together and transmitting a plurality of light signals having different wavelengths. EDFAs (Erbium-Doped Fiber Amplifiers) are broadly employed in the optical amplifiers for use in VDW systems.
An EDFA, an optical amplifier using an erbium (Er3+)-doped fiber (EDF) as an amplification medium, allows an optical signal to travel by illuminating the excitation light outputted from an excitation light source (semiconductor laser) to the optical fiber, and amplifies the level of the light signal by means of stimulated emission occurring at that time.
As a conventional optical transmission technique, a proposal has been made on a technique that, if a change in level of a light signal to be inputted to the optical amplifier is detected, an optical-surge occurrence alarm signal is outputted to reduce the gain of the optical amplifier. See for example Japanese Laid-open Patent Publication No. 2005-192077 paragraphs [0010] to [0013], FIG. 1.
The EDFA is able to amplify collectively and send a WDM signal in which a plurality of wavelengths are multiplexed together, by inputting excitation optical power commensurate with the number of wavelengths multiplexed. However, where adjustment control of excitation optical power cannot swiftly cope with the change in level of the light input to the EDFA (change in the number of wavelengths multiplexed in a WDM signal to be inputted, interruption of input light, or the like), a remaining portion of light, called an optical surge, arises, which has been responsible for the occurrence of transmission errors.
FIG. 21 is a figure showing an optical surge. It illustrates one example of optical surge occurrence. For example, it is assumed that 40 waves of a WDM signal are inputted to an EDFA and the number of wavelengths in the input WDM signal changes into one during transmission of those waves by providing excitation optical power for amplifying the 40 waves of one WDM signal.
If the light input to the EDFA changes in level due to such a change in the number of wavelengths multiplexed and control of switching from excitation optical power corresponding to 40 waves to that corresponding to one wave is slow, the EDFA requires that the time for which excitation light is inputted for 40 waves is increased relative to that of input for one wave. If this is the case, the gain abruptly changes with the result that a high-level optical surge, for example 1 ms or greater, instantaneously emerges from the EDFA.
There has been a problem that an optical surge, if transmitted, raises transmission errors or, in the case where EDFAs are connected in multiple stages, the receiver is broken by the amplification of cumulated optical surges.
Meanwhile, in the prior art of Japanese Laid-open Patent Publication No. 2005-192077, optical surge suppression is performed by reducing the gain of an optical amplifier under circuit control after detecting a change in level of the input light. However, because optical input power changes in level instantaneously, there is a difficulty in following changes in level under such control. In the gain adjustment under circuit control after a change in level of input light as in the prior art, an instantaneously occurred optical surge possibly propagates downstream, making it difficult to fully suppress an optical surge.